House of Mystery Huge List
In the early 1950's, the comic book industry was relatively new. It was an experimental time, when many different genres were being explored. Romance, war, westerns, sci-fi and horror books were all being put out by various companies. At the time DC was searching for new directions, so they decided to try their hand at the horror genre. It was doing very successful at EC comics, and several other companies had horror titles out at the time as well.
So it was that the first issue of House of Mystery came out, December 1951/January 1952. It had no host, and would feature several short, spooky stories that usually ended with a logical explanation for the supernatural. The title did relatively well, and ran with this formula until the late 1950's. Somewhere, a little bit before its hundredth issue, it went sci-fi. This is around the same time everyone's favorite doctor, Fredric Wertham, caused an uproar with his infamous book, Seduction of the Innocent. Soon after its release, the Catholic Comics Code appeared, causing hassles for the entire comics industry, and pretty much putting EC comics out of business. This isn't fact, but my opinion, that rather than deal with the negative publicity of a horror title, DC decided it would simply be easier to make the switch to sci-fi.